1. Field of the Invention
The present invention relates to the field of mammography, and more specifically, it relates to computerized analysis and compression of digital mammograms.
2. Description of Related Art
Detection and diagnosis of microcalcifications in mammograms is one of the most important and difficult tasks performed by radiologists. Early diagnosis and treatment of breast cancer, a leading cause of death in women, significantly improves the chances of survival. X-ray mammography is the only screening procedure with a proven capability for detecting early-stage, clinically occult breast cancers. Between 30% and 50% of breast carcinomas demonstrate microcalcifications on mammograms, and between 60% and 80% of breast carcinomas reveal microcalcifications upon microscopic examination. Therefore any increase in the detection of microcalcifications by screening mammography will lead to further improvements in its efficacy in the detection of early breast cancer in asymptomatic women. The American Cancer Society has recommended the use of mammography for screening of asymptomatic women over the age of 40 with annual examinations after age 50. For this reason, mammography may eventually constitute one of the highest volume X-ray procedures routinely interpreted by radiologists and with the highest volume of normals. Computer-aided detection and diagnosis may save the human radiologist time and tedium, especially on the "normal" images, so that more concentration can be dedicated to those cases that are clinically significant. The digital data will, however, require enormous amounts of storage space, and if full mammograms are stored at the highest possible resolution, less than 1% of the data will be clinically significant.
U.S. Pat. No. 4,907,156 describes a method and system for enhancement and detection of abnormal anatomic regions in a digital image. Classification schemes are used that are based on human-developed tests and thresholds which were designed to compromise on the number of true positives detected in order to minimize the number of false positive detections. The patent subtracts a signal-to-noise ratio (SNR)-suppressed image from a SNR-maximized image, but does not address the issue of compression at all. For signal extraction, the invention detects the boundary of the breast and performs a signal search only within the detected boundary. The classification scheme is based on pre-selected values determined from the clinical experience of radiologists and empirically by processing a number of test mammograms with known microcalcifications. As more digital radiographic imaging systems are developed, computer-aided searches become feasible. Successful detection schemes could eventually be hardware implemented for on-line screening of all mammograms prior to viewing by a physician.
Several investigators have attempted to classify and analyze detected mammographic abnormalities with digital computers. It is not clear whether these studies have achieved an accuracy acceptable for clinical practice, especially in the classification of microcalcifications. This failure can be attributed primarily to a large overlap in the features of benign and malignant lesions as they appear on mammograms.
The currently accepted standard of clinical care is such that biopsies are performed on 5 to 10 women for each cancer removed. Only with this high biopsy rate is there reasonable assurance that most mammographically detectable early carcinomas will be resected. Given the large amount of overlap between the characteristics of benign and malignant lesions on mammograms, computer-aided detection rather than characterization of abnormalities may eventually have greater impact in clinical care. Microcalcifications represent an ideal target for automated detection, because subtle microcalcifications are often the first and sometimes the only radiographic findings in early, curable, breast cancers, yet individual microcalcifications in a suspicious cluster (i.e., one requiring biopsy) have a fairly limited range of radiographic appearances.
The high spatial-frequency content and the small size of microcalcifications require that digital mammographic systems provide high spatial resolution and high contrast sensitivity. Digital mammographic systems that may satisfy these requirements are still under development. Digital radiographic systems with moderately high spatial resolution are made possible by fluorescent image plate/laser readout technology. Currently, digital mammograms with high resolution can be obtained by digitizing screen-film images with a drum scanner or other scanning system. The increasing practicability of digital mammography further underlines the potential ability of a computer-aided system for analysis of mammograms.
For electronically transmitting, storing and analyzing radiographs, mammograms remain the most challenging of all x-ray images because of the high resolution that is needed to accurately depict microcalcifications which are one of the three discrete abnormalities that indicates breast cancer. As a result, digitized mammograms can require about 50 megabytes of storage space per view, with 4 views per examination.
Data size reduction will be vital for the practical implementation of teleradiology systems (especially telemammography), since image size affects storage space, computer analysis time and transmittal times. Conventionally, there are lossless compression schemes which compress only at a ratio of 3:1, or there are lossy schemes which can compress to 30:1, but which must be tested exhaustively before they will be accepted in the medical community. Also, these schemes take time for compression as well as decompression. Image retrieval (even for viewing) is significantly slower if decompression must first take place.
A method of detecting microcalcifications indicating breast cancer is desirable. Such a method should include a reduction scheme which eliminates unnecessary information from the enormous amount of data in digital mammograms. It should achieve a reduction ratio of 10:1 to 30:1 without loss of appropriate resolution and with zero decompression time. It should use the context-based results of computer analysis to determine which areas of the mammogram must retain the highest resolution. All regions of the reduced mammogram should be capable of storage in a form that is ready-to-use and therefore does not impede retrieval times or viewing speeds. The present invention provides these advantages.